The majority of off-patent drugs prescribed to children lack dosing information specific to this population, which can result in treatment failures and toxicities. Dosing information is lacking because clinical trials involving children pose specific challenges, among the most important of which is the large number of children required to estimate appropriate dosing at every stage of child development. Population physiologically based pharmacokinetic (PBPK) models are mathematical tools that incorporate changes of childhood, along with drug characteristics and genetic variances, to find the most appropriate dose for every pediatric age. By incorporating these factors, PBPK models can reduce the number of children needed for clinical trials. This proposal will systematically develop, and prospectively validate, population PBPK modeling in children.